The present invention relates to electric power plants and more particularly to apparatus and methods for starting and operating such plants with automatic synchronization.
In the operation of an electric power plant, the prime mover for each plant generator is typically a steam turbine or gas turbine which is controlled in its operation to drive the electric generator from rest or turning gear speed to the generator running speed. The control system may be an electrohydraulic or an electropneumatic system employing an analog and /or digital electronic control or a digital computer control. If the electric power plant is included in a power system to which it is contributing power for distribution to various user points, a breaker is operated to connect the generator to the system when the generator acquires the proper operating status for synchronization. It is generally required that the generator speed be within a predefined range to provide for substantial matching of the generator electrical frequency and the power system electrical frequency, that the generator voltage magnitude be within a predefined range to provide for substantial matching of the generator and power system voltage magnitudes, and that the phase different between the generator voltage waveform and the power system voltage waveform be approaching zero for breaker closure substantially at the zero or coincident phase relationship between the two waveforms.
The synchronization conditions just described are needed to avoid generator damage and to avoid serious electrical disturbances in the electrical power system. It is desirable that the synchronization conditions be satisfied accurately and reliably for equipment protection and power systemsecurity purposes. Further, it is desirable that generator and breaker operation be controlled to provide fast synchronization, especially in gas turbine and other electric power plants where fast startup is needed to provide fast power contribution to the power system for power system security against power outage. The combination of startup reliability and synchronization speed is a measure of power plant availability for power generation which is especially significant in relation to gas turbine and other standby electric power plants.
One conventional synchronizing scheme is that relying principally on manual operations. Thus, a skilled plant operator typically may employ a synchroscope which provides a visual indication of the amount of phase difference and the slip or rate of change of phase difference between the generator and system bus or line voltages. If the slip is too great, raise or lower speed control action is applied to the prime mover as required by the operator. Concurrently, the operator makes any generator voltage regulator adjustments needed for voltage matching. When the generator and system voltage magnitudes are appropriate as indicated by meters and when the slip frequency and the relative phases are observed to be appropriate, the operator initiates a breaker close signal which typically operates a breaker closing relay coil. Normally, the operator generally anticipates the breaker closing time to provide for breaker closure as the two voltage waveforms are approaching phase coincidence or at the time point of phase coincidence.
In other prior art applications, automatic power plant generator synchronization has also been provided with varying degrees of automation and with varying kinds of hardware combinations. One scheme has involved the use of separate relay controls for the voltage matching, speed matching and synchronization functions. Generally, in the relay synchronization system scheme, the phase difference between the generator and system voltage waveforms is detected by the use of circuitry which vectorially adds the voltages and rectifies the sum. One relay is operated at a fixed phase angle and another relay is operated at a fixed breaker closing time ahead of a synchronization time point predicted from the envelope of the rectified phase difference voltage.
If the phase relay operates after the fixed time relay, the slip frequency is too fast and breaker closure is prevented. On the other hand, phase relay operation followed by operation of the fixed time relay signifies an appropriate slip frequency for synchronization and the breaker closing command is thus generated upon closure of the fixed time relay. Voltage and speed matching functions supportive to the functions of the relay synchronization system may be manually controlled or as already indicated automatically controlled by a separate voltage matcher and a separate speed matcher system. In some cases, a synchro-verifier may also be included to provide an independent check on the generator and power system conditions so as to prevent synchronization where conditions so warrant even though the automatic synchronization system may otherwise be calling for synchronization.
More recently, solid state automatic synchronization systems have been developed to provide substantial improvement over the earlier pertaining art relative to certain characteristics including synchronization accuracy and synchronization speed. The aforementioned Westinghouse patent applications pertain to such systems. In addition, a September, 1968 Westinghouse Engineer article entitled "Generators Sychronized Rapidly and Accurately by Automatic System" is also related to the same subject matter area. It is noteworthy at this point that in referencing prior art publications or patents or patent applications as background herein, no representation is made that the cited subject matter is the best prior art.
In the synchronization system of the Westinghouse Engineer article, a common package is modularly constructed to provide the synchronizer function and to provide at the user's option the voltage and speed matching functions as well as certain other system functions. The voltage matcher employs semiconductor circuitry in comparing the magnitudes of the generator and bus voltages and in generating corrective generator voltage adjustment signals. Among other features, there is included in the voltage matcher circuitry a capability for adjusting the accuracy with which the two voltage magnitudes are to be matched.
To provide speed matching, the prior art solid state synchronization system employs circuitry which becomes operable when the generator frequency is within .+-.10% of the bus frequency. Speed raise or lower signals are automatically applied to the separately provide prime mover speed control such that the time intervals between correction pulses become increasingly longer as the speed of the generator approaches the value corresponding to the electrical frequency of the bus. A capability is provided in the speed matcher for varying the closure time of the relays employed to generate the raise and lower speed control signals. Adjustability is also provided in the length of time during which the speed matcher will allow the generator to remain in a synchronous but phase difference condition.
A basic synchronizer component of the solid state synchronization system becomes operational if an optional voltage acceptor component indicates that both the generator voltage and the bus voltage are within predetermined limits. Solid state circuitry is employed to develop a triangular waveform which represents the varying phase difference between the generator and bus voltages. The breaker is signaled to close at an advance angle prior to phase coincidence to allow for the closure time of the breaker and the required advance angle is determined from the rate at which the generator and the bus voltages are approaching synchronism. If the determined advance angle is greater than a preset value, lock-out is provided against generation of a breaker closure signal until the slip frequency is reduced to a value within the acceptable range.
Background information relative to gas turbine electric power plants also pertains to certain aspects of the present invention. However, that background information is more fully considered in the aforementioned copending application Ser. No. 082,470.
The described and other known prior art synchronization systems and techniques have been characterized with certain disadvantages although they have been satisfactory under some performance and cost standards of evaluation. In the first place, manual synchronization tends to be undesirable since it depends upon the level of skill and judgment of the human operator. Further, the known existing state of the automatic synchronization art makes no provision for implementation of the automatic synchronization functions with a digital computer in those increasing numbers of applications where presently available digital computer hardware is economically justified and already available or scheduled for use for other electric power plant control functions. Thus, no provision is available to forego the added hardware cost of an automatic synchronization system in a power plant having a digital control computer.
It is also noteworthy that the computer advantages of extended control and operator performance flexibility are not available with prior art systems. Control flexibility does exist to some degree in the solid state systems and probably to a lesser degree in earlier systems, but as in all hard wired systems such flexiblity is necessarily relatively limited by practical considerations of economics, design and redesign. In at least some prior art systems and possibly most or all prior art systems, synchronization performance has also been relatively limited in efficiency, accuracy, reliability, speed and frequency range of operation.
Existing automatic synchronization systems have also been limited from the standpoint of system integration and the possibility of achieving increased system integration for extended quality and efficiency of performance. Thus, it is highly desirable in the continuous process of developing improved power plant operations that the overall plant control and the automatic synchronization system be tied together to provide better and more economic plant design and performance. The digital computer provides a unique capacity for enabling integration of system control and operation as the desirable characteristics of integration are made known by system development effort. However, prior art systems including gas turbine power plant systems have generally been limited in providing system integration and in providing extending possibilities for future system integration relative to automatic synchronization and general power plant system operations. System integration and digital computer implementation are particularly significant in gas turbine power plants where automatic remote operation and high availability are highly desirable.